C182 PLT HAS XMITTER FAILURE; SQUAWKS 7500 INSTEAD OF 7600.

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I DEPARTED I77 (BLUE ASH) ON AN IFR FLT PLAN TO AGC. AT THE TIME OF DEP; THE WX WAS VFR. THE FORECAST FOR I77 AND THE RTE OF FLT WAS FORECAST FOR IFR IN ANTICIPATION OF FRONTAL PASSAGE AND ANTICIPATED RAIN WITH POSSIBLE TSTMS. THE CLRNC WAS RECEIVED 'AS FILED' (DEPART I77/MOAKS INTXN/V5/APE/V12/AGC) WITH INSTRUCTIONS 'CLB TO 3000 FT EXPECT 5000 FT AFTER DEP. SQUAWK XXXX DEP FREQ 121.0.' DEP FROM I77 OCCURRED AT APPROX XA16. STATIC AND A LACK OF RADIO 'TFC' AFTER DEP LED TO AN ATTEMPT TO RAISE DEP AND CONFIRM THAT WE WERE STILL IN COM. THIS WAS TO NO AVAIL. I MADE THE SWITCH TO THE #2 RADIO AND IMMEDIATELY HEARD DEP TRYING TO CONTACT US. I TRIED TO XMIT BACK ON THE #2 RADIO; BUT A REPEAT REQUEST FROM DEP INDICATED THAT THEY WERE NOT RECEIVING US. SHORTLY AFTER MY ATTEMPT TO COMMUNICATE; DEP XMITTED THE REQUEST THAT IF I WERE RECEIVING THEM TO 'IDENT.' I COMPLIED. DEP'S RESPONSE WAS THAT THEY 'GUESSED' THAT WOULD CONTINUE TO COMMUNICATE WITH ME IN THIS FASHION THROUGHOUT THE FLT. SEVERAL ATTEMPTS WERE MADE TO XMIT OVER BOTH RADIOS TO SEE IF WE COULD RECTIFY THE PROB AND CONTINUE THE FLT. THIS DID NOT WORK. BOTH XMITTERS WERE INOP. GIVEN THAT WE WERE ONLY A SHORT DISTANCE INTO THE FLT; I DECIDED THAT THE SAFEST AND LOGICAL OPTION FOR US WAS TO RETURN TO OUR POINT OF ORIGIN FOR REPAIRS. TO ALERT DEP TO MY INTENTIONS; I TURNED TO THE XPONDER TO SQUAWK THE 'RADIO FAILURE' CODE PRIOR TO INITIATING THE HDG CHANGE FROM V5 BACK TO BLUE ASH. INSTEAD OF 7600; I MISTAKENLY SELECTED 7500. AFTER APPROX 3 MINS FOLLOWING THE TURN TOWARD BLUE ASH; DEP ASKED IF THIS WAS WHAT I WANTED TO SQUAWK TO 'IDENT.' I COMPLIED. I WAS THEN TOLD BY DEP TO MAINTAIN 3000 FT. ALL COMS FROM ATC FROM HERE ON OUT CEASED. THE IMPROPER IDENT CODE WAS NOT REALIZED UNTIL I WAS APCHED BY POLICE OFFICERS AFTER ENG SHUTDOWN. I THEN PHONED ATC TO EXPLAIN TO THEM WHAT HAD HAPPENED.